Thermoplastic containers and other foam products are often made of polystyrene sheet materials. U.S. Pat. Nos. 4,832,885 (Gross) and 5,032,106 (Warburton) disclose typical prior methods and equipment for thermoplastic molding of containers.
Automatic thermoforming equipment designed to form a one or more structures simultaneously are often large units of equipment which are designed for continuous operation. This equipment utilizes male and female mold members which define the thermoformed structures. Each half of the mold is a massive machined tool member which must stand up to repetitive thermoforming operations which not only involve physical force but rapid and wide changes in temperature. Failure of even the smallest component of the mold structures will cause a shutdown of the thermoforming apparatus.
Thermoplastic articles may be advantageously formed with perforated portions which are may be torn along a predetermined line. For instance, foamed styrenic polymer, such as general purpose homopolymer, high impact polystyrene or copolymer resins may be thermoformed to produce food service disposable containers with such features as tear-off lids or punch-out steam vents. Foamed sheet material having transverse perforated tear lines are useful as insulation baffles.
Such perforated articles are conventionally made in a two step process with thermal shaping followed by a separate cutting step. This requires significant investment in equipment and complicates manufacturing methods.
A novel thermoforming technique has been found which provides for simultaneous thermoforming and perforation of a thermoplastic web, such as oriented polystyrene, which may be foamed opaque or clear.